(1) Field of the Invention
This invention relates to propellors for watercraft. The term "propellors" shall also include marine screws and like propulsion units.
(2) Prior Art
Conventional propellors have two or more blades fitted to a hub and set at a pitch angle selected as most appropriate for the intended application. The propellors produce a divergent cone of thrust which reduces their efficiency, as the thrust is dissipated in the surrounding water and they also produce a "cocktail" or "roostertail" spray above the waterline, especially at higher speeds, indicating that potential thrust energy has been wasted. The divergent coning effect has been partially eliminated by producing shrouded or ringed propellors but these generate increased turbulence and suffer increased drag. Finally, the known propellors produce very little, if any, reverse thrust which can be used to reverse or brake the vessel to which the propellors are fitted.
Various types of propellors have been proposed. For example, U.S. Pat. No. 2,087,243 (Caldwell) discloses a propellor of convergent radial height along the propellor, where the forward portion of each blade is forwardly inclined and the rearward portion is rearwardly inclined and where the line of junction of the blade with the hub is "a regular spiral of a given pitch, the peripheral pitch of the blade being greater than that of the junction line". United Kingdom Patent No. 8568 of 1909 (Marks) discloses a propellor where the peripheral pitch of the blade increases along the propellor and the radial height of the blade increases in height along the propellor.
Even these propellors have not proved satisfactory in overcoming the problems with the known propellors discussed above.